1. Field of the Invention
The present invention relates to the field of field effect transistors and more particularly to a field effect transistor having a pair of source/drain regions formed from a narrow bandgap semiconductor film and its method of fabrication.
2. Discussion of Related Art
Integrated circuits, such as microprocessors, digital signal processors, and memory devices are made up of literally millions of transistors coupled together into functional circuits. An example of a conventional metal oxide semiconductor field effect transistor (MOSFET) 100 is illustrated in FIG. 1. Transistor 100 includes a gate electrode 102 formed on a gate dielectric layer 104 which in turn is formed on a monocrystalline silicon substrate. A pair of sidewall spacers 108 are then formed along laterally opposite sidewalls of the gate electrode 102. A pair of source/drain regions 110 are then formed along opposite sides of the gate electrode 102 as shown in FIG. 1. The source and drain regions comprise heavily doped portions of the silicon substrate 106. Typically, a silicide layer 112, such as titanium silicide or nickel silicide, is used to couple contacts 120 formed in a interlayer dielectric 140 to the source and drain regions 110. Silicide regions 112 are generally formed by alloying a metal, such as titanium, nickel or cobalt with the silicon substrate 106 to form the metal silicide. Additionally, contacts 120 are generally formed from a relatively high resistance film such as tungsten which can be conformally deposited so that it fills contact opening formed in the dielectric layer 140.
The dimensions of transistor 100 are continually being scaled down in order to increase packing density and thereby increase the computational power of the fabricated integrated circuits. Unfortunately, as transistor 100 is continually scaled down, the external resistance of the device (Rext) is increased degrading device performance, such as its drive current. Presently, the problem of increased Rext is solved by high active doping of the source and drain region and fully siliciding the source and drain regions. High active doping of the source and drain regions can decrease the electron mobility in the source and drain regions. Fully siliciding the source and drain regions results in a schkotty barrier transistors resulting in ambipolar conduction. Additionally, formation of silicide films by alloying a metal and the semiconductor substrate together can increase the thermal budget of the device which can decrease device performance.